Over two billion years ago, a single-celled organism engulfed a bacterium that could use oxygen to produce energy. The bacterium was protected by the bigger cell, while the bigger cell harnessed the bacterium's energy-producing superpower. Those oxygen-consuming bacteria are now called mitochondria and their DNA is so important that it is now encoded in the larger cell's nucleus. This mutually beneficial relationship is the hallmark of all eukaryotic, multicellular organisms. Or so we thought.
Recent work from Dayana Yahalomi from Tel Aviv University, Israel, and a group of international researchers from the USA, France and Canada, has revealed that a cnidarian (a relative of jellyfishes) called Henneguya salminicola, which also parasitizes salmon, has completely lost its mitochondrial genome. In other words, this organism is lacking the very machinery that scientists believed is part of what makes a eukaryote … well, a eukaryote: the ability to carry out aerobic cellular respiration.
The project began with Yahalomi and colleagues deciding to look for the genes that make mitochondria and mitochondrial proteins in H. salminicola spores. However, when they sequenced the cnidarian's genome, they couldn't find any of the essential genes that they had expected to find. So, to make sure that their experiments and analysis were working correctly, the researchers tested their techniques on a relative of H. salminicola, another parasite called Myxobolus squamalis, which is known to have mitochondria and was expected to have the mitochondria-coding genes in its DNA. Sure enough, after isolating and sequencing M. squamalis DNA, they found that the parasite had mitochondria-coding genes, which led them to the mind-boggling conclusion that the parasitic H. salminicola has done away with its mitochondria. However, when the authors used electron microscopy, they found that H. salminicola possess small cellular components that resemble mitochondria, called mitochondria-related organelles.
Intrigued by H. salminicola’s alternative to genuine mitochondria, the team took a closer look at the organelles using microscopes. They found that the cnidarian's mitochondria-related organelles did not resemble the organelles found in anaerobic, single-celled organisms. Instead, they looked a lot like genuine mitochondria, including having the hallmark internal folded membranes, called cristae. In addition, the team found that the organelles possess proteins that are usually lacking in these organelles in other organisms. By delving more deeply into the DNA of this cnidarian, the researchers also found incomplete mitochondria genes, called pseudogenes. Combining these observations, the authors concluded that H. salminicola lost their mitochondria relatively recently.
But why have H. salminicola lost their mitochondria and how are they able to ‘make’ energy without the essential structures? Yahalomi and her group suspect that the ‘why’ might have something to do with the cnidarian's lifestyle, which includes two periods when they reside within host organisms and are likely to experience extended periods without oxygen. The team suspects that instead of wasting energy-building mitochondria when no oxygen is available to fuel them, the parasite has simply done away with the structures.
Answering the question of how H. salminicola makes energy will take longer, because it is not possible to grow these animals in the laboratory. However, it is clear to Yahalomi and colleagues that losing their mitochondrial DNA and the ability to perform aerobic respiration has not hindered H. salminicola in any way, as they appear to thrive in marine, freshwater and terrestrial environments, which goes to show that, from the point of evolution, less can be more!